We figured out what the problem was, with the help of a guy from the City water department (image that!). It turns out that I had a hose outside that had the spicket turned on, but it had a sprayer on the other end. So, the water was backing up into the house, after being in the hose. Disconnecting the hose from the house, and letting the water flush for awhile seems to have done the trick. Thanks for your suggestions.